A high-pressure injection, and an injection valve (injector) equipped with a piezoactuator as the injection actuator, are described in German Patent Application Nos. DE 100 32 022 A1 and DE 100 02 270 C1. An injection valve of this kind serves for precisely regulatable fuel metering into the combustion chamber of the internal combustion engine.
In an injection valve of this kind, the piezoactuator serves to control the motion of a nozzle needle of the injection valve, either the nozzle needle itself or a control valve controlling the motion of the nozzle needle being triggered.
For exact metering of fuel into the combustion chamber, the most accurate possible knowledge of the stroke length of the piezoactuator or nozzle needle, in interaction with the control valve, is necessary. As is evident from FIG. 1, in the piezo common rail (PCR) systems described in German Patent Application No. DE 100 02 270 C1, the control valve is actuated via the piezoactuator and an interposed hydraulic coupler, the valve in turn controlling the nozzle needle motion by modulation of the pressure in a so-called control chamber.
The pulsed triggering voltage of these piezoactuators that is required for a specific injected quantity depends, as is conventional, on state variables of the injection system such as, for example, the rail pressure instantaneously present in a common rail, or the temperature of the piezoactuator. A corresponding adaptation of the triggering voltage must therefore take place in order to make possible very small injected quantities. The aforesaid dependence on the rail pressure results from the aforementioned manner of operation of the injection valve, and the aforesaid temperate dependence from the change in the stroke length of the piezoactuator with temperature. The effect on injected quantity results from the difference in actual triggering onset and triggering end with varying actuator stroke length or with varying hydraulic and mechanical operating parameters.
In addition to the aforementioned state variables, there are also sample-to-sample variations in particular in the actuator stroke length, and variations in the function of the hydraulic coupler, in the control valve seat, and the like.
Conventionally, the aforesaid effects are taken into account in the context of a “worst-case” evaluation performed on a steady-state basis, i.e., they cannot be taken into account in the context of an activation occurring during operation of the internal combustion engine. It is therefore not possible to improve the accuracy of the injected quantities even further during operation. This is disadvantageous specifically with regard to emissions standards that must be met in the future.
German Patent No. DE 39 29 747 A1 describes a method for controlling a fuel injection system having a high-pressure fuel pump, the fuel quantity to be injected into the respective combustion chambers of the internal combustion engine being controlled by means of solenoid valves. Production- and aging-related variations in the fuel quantity injected into the individual combustion chambers cause different fuel quantities to be delivered for the same triggering signal, resulting, in particular with very small quantities injected in preinjection operations, in considerable quantity errors. To avoid these variations, in certain operating states of the internal combustion engine a determination is made of the pulse duration of the triggering pulses of the solenoid valve at which a preinjection is currently beginning. Based on the triggering pulse duration thus determined, equalization signals for the triggering pulses are created and are permanently stored.